Additive manufacturing or rapid prototyping methods for producing objects comprise layer-by-layer solidification of a material, such as a metal powder material, using a laser beam. A powder layer is deposited on a powder bed in a build chamber and a laser beam is scanned across portions of the powder layer that correspond to a cross-section of the object being constructed. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required. In a single build, more than one object can be built, the objects spaced apart in the powder bed.
During the melting or sintering process, debris (e.g. condensate, unsolidified particles of powder etc) is produced within the build chamber. It is known to introduce a gas flow through the build chamber in an attempt to remove debris from the chamber in the gas flow. For example, the M280 model of machine produced by EOS GmbH, Munich, Germany comprises a series of gas outlet nozzles located to the rear of the powder bed that pass a flow of gas to a series of exhaust vents that are located at the front of the powder bed. In this manner, a planar layer of gas flow is created at the surface of the powder bed. A similar arrangement is provided in Renishaw's AM250 and AM125 machines, wherein apertures at either side of a powder bed provide substantially planar gas flow across the powder bed. It has been found that debris can be blown from one section of an object to another section of the object or to another object. This can result in non-uniformity and increased porosity of the solidified metal layers generated by the melting process. In particular, debris blown across the powder bed can result in an increase in surface roughness such that pores are formed between adjacent layers. Non-uniformities in a build can result in an object not conforming to the desired design and damage the apparatus. In particular, a wiper blade is typically used to spread each powder layer across the powder bed. Solidified structures that project out of the powder bed can catch on and cause damage to the wiper blade. Damaged wiper blades may result in powder layers with ridges of powder. Accordingly, non-uniformities in a build may be a concern not just for the layer being formed, but powder layers formed thereafter.